When a lightning strikes a wind turbine, the lightning often hits an area of one or more of the blades from where the current from the lightning strike is conducted inside or outside the blade by conducting means, like a down conductor, to one or more wind turbine components like the hub, the nacelle and the tower and further down to the ground. The aim is though to ensure that the current from the lightning strike on its way through the wind turbine does not reach any electrical components like the generator or converter or other critical components like the main bearings or gearbox that would be damaged by the large energy of the lightning strike.
The combination of a high voltage, large electrical current and strong resulting electromagnetic fields from the lightning strike could damage the generator or other electrical components of the wind turbine. Having a direct drive wind turbine, also called a gearless wind turbine, makes it is more difficult to conduct the lightning strike from one or more of the blades through the wind turbine without damaging especially the generator as it is connected directly to the blade hub of the wind turbine. Having a direct drive wind turbine with a generator with an outer rotating rotor makes it even more difficult to find a way to conduct the electrical current from the lightning strike on the blades of the wind turbine to the ground without damaging the generator.
In state of the art design of a wind turbine, the lightning strike is conducted by the down conductor of the blades directly to the hub which will further conduct the current from the lightning strike to the nacelle and further down to the ground. This kind of lightning protection system does not work for a direct drive wind turbine where the generator is connected directly to the hub as the generator then might be damaged by a lightning strike.
U.S. Pat. No. 7,377,750 B1 shows a lightning protection system for conducting lightning strikes to the blades and further to the region surrounding a blade hub and along a path around the blade hub and critical components of the wind turbine. The lightning conducting path around a hub is quite long with one or more directional shifts or turns and every place where this path has a directional shift or turn there is a substantial risk that the electrical current from the lightning strike jumps from the lightning conducting path to a component close to the lightning conducting path, like e.g. the blade hub and then further to the generator.